Fasting Requirements for Cortisol and Thyroid Function Testing
No fasting is required for cortisol, free T3, or free T4 testing in routine clinical practice. 1
Cortisol Testing Requirements
Morning timing (around 8 AM) is essential for cortisol testing, but fasting is not necessary. 2, 1
- The Endocrine Society recommends blood collection in the morning around 8 AM for most accurate cortisol results, as cortisol follows a diurnal rhythm with peak levels in early morning 1
- For diagnostic evaluation of adrenal insufficiency or hypophysitis, morning ACTH and cortisol levels should be obtained preferably around 8 AM, but no fasting requirement is specified 2
- Patients should avoid heavy meals within 2-3 hours before testing and abstain from caffeine on the day of testing, but complete fasting is not required 2
Important Caveat on Fasting Effects
While fasting is not required for the test itself, be aware that prolonged fasting (>24 hours) can artificially elevate cortisol levels, which may confound interpretation 3, 4, 5:
- Both overfeeding and fasting increase circulating free cortisol levels and alter the cortisol-to-cortisone ratio 3
- Fasting-induced cortisol elevations can suppress TSH levels by approximately 51-65%, potentially masking thyroid dysfunction 5
- This effect is more pronounced in men than women 3
Free T3 and Free T4 Testing Requirements
No fasting is required for thyroid function testing, and morning timing is not critical. 1, 6
- Free T4 and free T3 measurements can be performed at any time of day without fasting 1
- The American Thyroid Association confirms that no special preparation is usually required unless the doctor specifies otherwise 1
- Free hormone measurements (FT4 and FT3) have superseded total hormone determinations due to higher diagnostic performance and are not affected by meal timing 6
Clinical Context Considerations
When evaluating suspected central hypothyroidism (low TSH with low free T4), always check morning cortisol levels simultaneously to rule out adrenal insufficiency before initiating thyroid hormone replacement 2, 1:
- Starting thyroid hormone replacement before ruling out adrenal insufficiency can trigger life-threatening adrenal crisis 1
- In hypophysitis evaluation, obtain TSH, free T4, ACTH, and cortisol together, preferably in the morning around 8 AM 2
Practical Testing Protocol
For routine outpatient testing:
- Schedule cortisol testing between 7-9 AM for accurate baseline values 2, 1
- Allow light meals or snacks before testing, avoiding only heavy meals within 2-3 hours 2
- Avoid caffeine on the day of testing (including caffeinated sodas) 2
- Free T3 and T4 can be drawn at the same time without additional preparation 1, 6